The present invention relates to compact disc recording, and more specifically, to compact disc recording at a slow speed.
In compact disk technology, sub-1xc3x97 recording, i.e. recording at a slower speed than reading speed, is used when real-time compression is used to reduce the data size for audio or video data. For example, a real time compression of audio data can result in a data rate that is as low as {fraction (1/10)}th of the uncompressed data. Real time video compression, such as MPEG-1 for VCD applications typically results in a 1xc3x97 data rate after compression. Newer compression techniques, such as MPEG-4, can achieve even higher compression ratios. Thus, if real-time compression is used, the recording speed of the data will be below 1xc3x97.
Slower recording speeds have the advantage of reduced power consumption due to reduced rotational speed and laser power. However, for CD-R media the Orange book specifies a laser power for writing at 1xc3x97 speeds (generally 1.2 to 1.4 meters/second tangential velocity of the recording head), and an equation to adjust the write power upwards for writing at higher speeds, at a multiple of the nominal velocity. However, no such equation is provided for writing speeds below 1xc3x97.
During a typical CD-R write operation, a mark is formed by switching the laser from read power to write power at the starting point of the mark, and switching back to read power at the ending point. Read power, typically an order of magnitude smaller than write power, is used to illuminate the disk between marks for tracking and focus servos. It is theoretically possible to write at sub-1xc3x97 speeds by linearly reducing the write power to some low level.
The recording layer of CD-R media is optimized and tested to perform at a specified range of write speeds. Outside this range, the relationship of mark formation, write power, and writing speed becomes non-linear and unpredictable. Although for real-time data compression it is desirable to record data at speeds as slow as 0.05xc3x97(0.06 meters/second) no known CD or DVD recorder, media, or optical recording head is specified for writing at speeds less than 1xc3x97, nor is there guidance given in the Orange book.
Recording hardware, the recording head as well as the circuits that control write laser power, are designed to work for writing speeds over 1xc3x97. If sub-1xc3x97 writing is attempted by simply reducing the write power, these elements may require substantial modifications. Writing at 1xc3x97 power for sub-1xc3x97 write speeds may damage the CD-R medium by overheating.
Therefore, a method of recording at sub-1xc3x97 writing speeds without requiring substantive modifications to the recording head and laser power controls is needed.
A method and apparatus for sub-1xc3x97 recording is described. The method of writing a mark on a compact disk (CD-R) medium spinning at a slow rate comprises varying the laser power between a write power and a read power while writing the mark, such that when read, the mark is read as a single mark.